Amplifiers heretofore used for MR sensors that have had differential inputs and differential outputs were unable to operate with a single power supply voltage of less than five volts due, among other things, to excessive power dissipation. Permalloy magnetoresistive (PMR) amplifiers either require dual power supply voltages with a grounded disk/spindle assembly or require that the disk spindle assembly float at a potential between ground and a single supply voltage. Care must be taken to not short the disk/spindle assembly to ground during manufacture; and the accuracy with which said assembly is biased relative to the potential of the MR sensor is critical, requiring expensive resistor adjusting and/or a dedicated feedback circuit including a capacitor external to an arm electronics (AE) chip.
There is a need for an improved amplification circuit and method for use with MR sensors in low end storage products that has the following desirable features:
(1) A single (grounded) power supply. PA1 (2) Up to 3 dB less noise than differential input amplifiers. PA1 (3) Operability with power supply voltages of five volts or less without requiring an electrically floating disk/spindle assembly. PA1 (4) Reduced power dissipation. PA1 (5) Continuously adjustable read bias current without resistor trimming or noise aggravation. PA1 (6) Fast activation and deactivation to minimize power dissipation between reading of sector servo patterns circumferentially spaced on a recording disk. PA1 (7) Current biased MR sensors with dR/R signal detection, as taught in U.S. Pat. No. 4,706,138, to render the amplifier output signal relatively insensitive to variations in stripe height of the MR sensors. PA1 (8) Use, with a single-ended amplifier, of a disk enclosure as a Faraday cage to enable interference-free amplification of MR sensor signals.